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355CAV fan making a whine and warbling sound

Hello. Last fall a reputable outfit replaced my old system with a: 355CAV042060, 186BNA018000, CNPVP2417ATA, and SYSTXBBUID01-C. The unit is in a room adjacent to the living room with a return pulling air from the living room. So I expect to, and can, hear the system operating. During the heating season I found it reasonably quiet (ran on low 99.99% of the time and medium only occasionally) and the blower fan made no strange noises. During this first cooling season, however, the blower fan has been making some annoying noises. I wanted to ask about this in a forum before calling the contractor.

When the air conditioner is running it sounds as though the blower fan RPM is frequently being changed by modest amounts. It sounds as though at some RPMs the fan produces a whine, at other RPMs the fan produces an oscillating "warbling" sound. Neither the whining nor warbling is very loud. However, they are clearly heard and as the fan RPM moves around the whining comes and goes, sometimes changing in pitch, and the warbling comes and goes. Which creates a constantly changing series of noises that is disruptive to quiet work, trying to take a nap, watching a quiet movie, those sorts of things. That is typical behavior especially on hot days. Occasionally, though, the whine/warble don't occur or at least aren't obvious. Unfortunately, such was the case during my summer checkout appointment. During which it was suggested I give a different config (constant low fan, efficiency, higher humidity) a try for a while. I thought the system was noisier after that (and also less comfortable) so I put it back to the original config (auto fan, comfort, default humidity) for now.

Does this noise I'm describing sound like something any of you have run into before? I'm not sure if there is anything I can do in terms of gathering more information before calling the contractor. If there is, though, I'd like to since this is a somewhat variable problem and often those aren't much fun to track down.

Hello. Last fall a reputable outfit replaced my old system with a: 355CAV042060, 186BNA018000, CNPVP2417ATA, and SYSTXBBUID01-C. The unit is in a room adjacent to the living room with a return pulling air from the living room. So I expect to, and can, hear the system operating. During the heating season I found it reasonably quiet (ran on low 99.99% of the time and medium only occasionally) and the blower fan made no strange noises. During this first cooling season, however, the blower fan has been making some annoying noises. I wanted to ask about this in a forum before calling the contractor.

When the air conditioner is running it sounds as though the blower fan RPM is frequently being changed by modest amounts. It sounds as though at some RPMs the fan produces a whine, at other RPMs the fan produces an oscillating "warbling" sound. Neither the whining nor warbling is very loud. However, they are clearly heard and as the fan RPM moves around the whining comes and goes, sometimes changing in pitch, and the warbling comes and goes. Which creates a constantly changing series of noises that is disruptive to quiet work, trying to take a nap, watching a quiet movie, those sorts of things. That is typical behavior especially on hot days. Occasionally, though, the whine/warble don't occur or at least aren't obvious. Unfortunately, such was the case during my summer checkout appointment. During which it was suggested I give a different config (constant low fan, efficiency, higher humidity) a try for a while. I thought the system was noisier after that (and also less comfortable) so I put it back to the original config (auto fan, comfort, default humidity) for now.

Does this noise I'm describing sound like something any of you have run into before? I'm not sure if there is anything I can do in terms of gathering more information before calling the contractor. If there is, though, I'd like to since this is a somewhat variable problem and often those aren't much fun to track down.

Thanks for your time.

Your system is over-sized or the ducts are to small or both. If the system ran 99.99 % of the time in low during the heating system and when it switches to cooling and the air flow triples, the variable speed motor begins to search for a lower static pressure that it can function at.

I would suggest you call the reputable company back and have them determine why the system is not performing.

Last winter in your area was exceptionally warm, so we can't really establish a fan noise speed of heating vs. cooling until you get some really cold weather that will have the furnace up on high fire. However, the noise issue may be the result of a return air grill. It's not uncommon to have a variable speed blower that pulls more air through the same grill, resulting in higher velocity and setting the grill fins to striking a nice harmony. If that's the case, it's pretty easy for anyone with a hand seamer to change the louver pitch and eliminate the noise. Did the installing company do a real Manual J load analysis before installing the equipment?

If YOU want change, YOU have to first change.

If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

I received Manual Js and discussed the subject with multiple companies. My home is very small and based on the heat load numbers (low to mid 20000s IIRC) the expectation was that this type of furnace would rarely if ever go above low stage. If my notes are correct this first winter's furnace cycles were... Low:7090, Med:34, High:10 and the hours were Low:973, Med:9, High:2. So for some reason it did even run on high at times during this past somewhat mild winter.

As for the AC, I believe that is a 1.5 ton. I was told my duct work was adequate for that and most said a 2-ton would be fine. I made some notes during operation (numbers displayed by Evolution control) which suggest that when the AC is running the airflow CFM is steady at 525 CFM, with blower RPM varying between 683-719 and static pressure varying between 0.36-0.37. The 20x20 return grille is not whistling or rattling or humming(*). These noises I describe are definitely coming from the unit. I'm obviously assuming it is the blower fan itself and some kind of mechanical and/or electrical noise.

* I've tried opening the grille and removing the filter. That causes the fan RPM to drop and become generally quieter. It seems to take the fan RPM out of the range where the noises I describe occur or are more pronounced. There is still some unevenness to the sound made by the fan which I could only describe as a more subtle version of the warbling sound.

I received Manual Js and discussed the subject with multiple companies. My home is very small and based on the heat load numbers (low to mid 20000s IIRC) the expectation was that this type of furnace would rarely if ever go above low stage. If my notes are correct this first winter's furnace cycles were... Low:7090, Med:34, High:10 and the hours were Low:973, Med:9, High:2. So for some reason it did even run on high at times during this past somewhat mild winter.

As for the AC, I believe that is a 1.5 ton. I was told my duct work was adequate for that and most said a 2-ton would be fine. I made some notes during operation (numbers displayed by Evolution control) which suggest that when the AC is running the airflow CFM is steady at 525 CFM, with blower RPM varying between 683-719 and static pressure varying between 0.36-0.37. The 20x20 return grille is not whistling or rattling or humming(*). These noises I describe are definitely coming from the unit. I'm obviously assuming it is the blower fan itself and some kind of mechanical and/or electrical noise.

* I've tried opening the grille and removing the filter. That causes the fan RPM to drop and become generally quieter. It seems to take the fan RPM out of the range where the noises I describe occur or are more pronounced. There is still some unevenness to the sound made by the fan which I could only describe as a more subtle version of the warbling sound.

The 525 would indicate they have your CFM set at 350 per ton. What is your geographical area?